This invention relates to a liquid crystal display device, in particular, to a liquid crystal display device mainly using an electric field substantially in parallel to a display screen.
In recent years, liquid crystal display devices have been used in various fields by virtue of their merits: lightweight, thinness, and low power consumption. Particularly, a widespread liquid crystal display device has a structure in which twisted nematic (TN) liquid crystal is held between electrode substrates.
In such a conventional liquid crystal display device, the brightness and color considerably vary with the viewing angle. This is a factor which makes it difficult to comply with a demand for a large display screen.
Under the circumstances, development of a liquid crystal display device mainly using an electric field substantially in parallel to a display screen has been continuing so as to solve the problem. Such a liquid crystal display device is disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 63-21907.
As shown in FIG. 9, the liquid crystal display device comprises an array substrate 10 having a pixel electrode 1 and a counter electrode 3 both formed thereon, a counter substrate 20 facing the array substrate 10, and a liquid crystal cell 30 which contains TN liquid crystal molecules of a positive anisotropic dielectric constant and is held between the substrates 10 and 20. The liquid crystal cell 30 is held between the substrate 10 and 20 via aligning films 13 and 23 which are treated to align the liquid crystal molecules in the same direction R. The alignment (treatment) direction R forms a predetermined angle xcex81, for example, 80xc2x0 with respect to a direction of an electric field E created between the pixel electrode 1 and the counter electrode 3.
Polarizing plates 40 and 50 are respectively mounted on outer surfaces of the substrates 10 and 20 to have a cross-Nicol system in which the polarization axis P1 of the polarizing plate 40 is set in the alignment direction R, and the polarization axis P2 of the polarizing plate 50 is set in a direction orthogonal to the alignment direction R.
With this system, the light transmittance is set at a minimum value when no voltage is applied between the pixel electrode 1 and the counter electrode 3, and at a maximum value mainly by the birefringence effect of the liquid crystal molecules aligned in the electric field direction E shown in FIG. 10 when a voltage of a sufficient level is applied between the pixel electrode 1 and the counter electrode 3.
In such a liquid crystal display device, upon application of the voltage, the alignment of liquid crystal molecules is twisted in a range from the main surface of each substrate to the middle of the liquid crystal cell since a binding force is applied to the liquid crystal molecules from the main surface of each substrate due to the alignment treatment.
Since a considerable period of time is required for resuming the twisted alignment obtained by application of a voltage to an initial alignment of the molecules, the display device has a drawback that the response speed is slow. This drawback is also raised in the case where the liquid crystal cell has a negative anisotropic dielectric constant.
On the other hand, Jpn. Pat. Appln. KOKOKU Publication No. 7-261152 discloses a technique of controlling the light transmittance by selection between application of a high level voltage and application of a low level voltage, instead of selection between application of a voltage and non-application of the voltage, so as to use ICs of a low withstand voltage in a liquid crystal display device. Since this technique enables reduction in the amplitudes of voltages applied to the electrodes, ICs of a low withstand voltage can be used in the liquid crystal display device.
The inventors of the present invention have studied this technique and found that the technique enhances the response speed as a result of the control of liquid crystal molecules during which voltage application is retained.
However, it is also recognized that the contrast ratio is deteriorated due to the above-mentioned control of liquid crystal molecules during which voltage application is retained.
The present invention has been achieved in view of the above-mentioned problem, and an object of the present invention is to provide a liquid crystal display device capable of attaining good viewing angle characteristics without deteriorating characteristics of the device, such as contrast ratio and response speed.
According to the invention, there is provided a liquid crystal display device which comprises first and second substrates; a liquid crystal cell held between the first and second substrates and containing liquid crystal molecules arranged to have an alignment corresponding to alignment properties of inner surfaces of the first and second substrates; first and second electrodes formed on the first substrate to apply a lateral electric field substantially parallel to the first and second substrate into the liquid crystal cell; first and second polarizing plates having individual polarization axes and mounted on outer surfaces of the first and second substrates, respectively; and an optical retardation plate interposed at least between the first polarizing plate and the first substrate; wherein an optical axis and retardation value of the optical retardation plate are determined to compensate for twisting of the alignment of the liquid crystal molecules caused upon application of the lateral electric field.
According to another aspect of the present invention, there is provided a liquid crystal display device which comprises first and second substrates; a liquid crystal cell held between the first and second substrates and containing liquid crystal molecules arranged to have an alignment corresponding to alignment properties of inner surfaces of the first and second substrates; first and second electrodes formed on the first substrate to apply a lateral electric field substantially parallel to the first and second substrate into the liquid crystal cell; first and second polarizing plates having individual polarization axes and mounted on outer surfaces of the first and second substrates, respectively; and first and second optical retardation plates interposed between the first polarizing plate and the first substrate and the second polarizing plate and the second substrate, respectively; wherein when a first lateral electric field is produced to obtain a dark state and a second lateral electric field is produced to obtain a bright state, optical axes and retardation values of the first and second optical retardation plates are determined to compensate for twisting of the alignment of the liquid crystal molecules caused upon application of the first lateral electric field.
In the liquid crystal display device of the present invention, a lateral electric field is used to switch the alignment of liquid crystal molecules, and thus can attain good viewing angle characteristics. Further, the optical axis and the retardation value of each optical retardation plate is determined to compensate for twisting of the liquid crystal molecules, and thus the contrast ratio can be improved while maintaining a high response speed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.